From NPR’s All Tech Considered:
Godwin explains that the comparison is usually out of desperation. “Discovering what other people think when they disagree with you is quite disturbing. So there’s a tendency to escalate,” he says. He adds that the Internet created the first opportunity for such a diverse range of people to interact in an unmediated space.
To this I would add that dealing with disagreement is time-consuming, fatiguing, and emotionally distressing. Finding an easy way to do it is the name of the game. Hitler will do just fine.